1. Field of the Invention
The present invention relates to a hydraulic actuator control device and a hydraulic actuator control method. In particular, the invention relates to a hydraulic actuator control device and a hydraulic actuator control method used in a variable valve timing mechanism that variably controls the opening and closing timing of an intake valve or an exhaust valve of an internal combustion engine.
2. Description of the Related Art
In a variable valve timing mechanism, a hydraulic actuator is used to change the displacement angle of a cam shaft relative to a crank shaft. The hydraulic actuator is provided with two oil chambers, i.e., an advance-side oil chamber and a retard-side oil chamber. The valve timing is advanced by supplying pressurized oil to the advance-side oil chamber and discharging the pressurized oil from the retard-side oil chamber, and is retarded by supplying the pressurized oil to the retard-side oil chamber and discharging the pressurized oil from the advance-side oil chamber.
The supply and discharge of the pressurized oil to and from the two oil chambers of the hydraulic actuator is controlled by an oil control valve (OCV). The oil control valve controls the supply and discharge of the pressurized oil depending on the position of a spool within a sleeve. When the spool stays in a neutral region within the sleeve, the two oil chambers are prevented from communicating with a hydraulic pump and an oil tank. If the spool moves from the neutral region to an advance side, the advance-side oil chamber is connected to the hydraulic pump, and the retard-side oil chamber is connected to the oil tank. If the spool moves in a direction opposite the advance side (i.e., to a retard side), the retard-side oil chamber is connected to the hydraulic pump, and the advance-side oil chamber is connected to the oil tank. The spool is driven by a solenoid, and the position thereof is controlled by the value of the duty current that is output to the solenoid.
In the oil control valve, the neutral region within the sleeve has a specified width. When the spool moves within the neutral region, the supply and discharge of the pressurized oil to and from the two oil chambers is minimal. For this reason, in the variable valve timing mechanism, a dead zone in which the valve timing does not respond to or shows reduced responsiveness when the duty current value changes exists near a duty that makes the supply amount of the pressurized oil nearly zero, i.e., a duty that holds the current valve timing.
When advancing the valve timing, the duty that is output to the control valve is changed from a holding duty to an increased duty. In contrast, when retarding the valve timing, the duty that is output to the control valve is changed from the holding duty to a decreased duty. At this time, a valve timing changing speed is kept small until the duty gets out of the dead zone. As soon as the duty gets out of the dead zone, the valve timing starts to be rapidly changed in accordance with the duty value. In this way, presence of the dead zone heavily affects controllability of the valve timing.
Japanese Patent Application Publication No. JP-A-2003-336529 describes a technique for learning the upper and lower end values of a dead zone during controlling valve timing. With the technique described in the Japanese Patent Application Publication No. JP-A-2003-336529, the duty when the actual value of the valve timing begins to be changed toward a target value of the valve timing in response to a change in the target value is learned as the upper or lower end value of the dead zone.
Variations due to individual differences of control valves exist in control characteristics of a variable valve timing mechanism, i.e., changing tendency of responsiveness of valve timing to a change in duty. Even within an individual control valve, variations in control characteristics occur depending on an oil temperature or other conditions. In order to accurately control the valve timing, it is necessary to precisely determine the control characteristics of the variable valve timing mechanism and then to decide the duty to be output to the control valve, based on the control characteristics thus determined.
According to the related art, the upper and the lower value of the dead zone or the holding duty may be determined by conducting learning through valve timing control. Therefore, it is believed that accurate duty control may be executed within the dead zone. However, because accurate determination of the control characteristics outside the dead zone is not conducted in the related art, there is no choice but to leave the duty control outside the dead zone as it stands.